1. Field of the Invention
The present invention relates to a pilot signal extracting circuitry designed so that, from a composite signal consisting of plural signals modulated by a suppressed carrier modulation system, is extracted a pilot signal for controlling the generation of a carrier wave which is used for the demodulation of the composite signal.
2. Brief Description of the Prior Art
In demodulating a carrier-suppressed modulated wave (a wave modulated by the suppressed carrier modulation system), it is required to generate a carrier wave which is synchronous with the carrier wave used in the modulation. For this purpose, generally there is added, to the carrier-suppressed modulated wave, a control signal or the so-called pilot signal for controlling the generation of a carrier wave which is used for the demodulation of the carrier-suppressed modulated wave.
The modern FM stereo broadcasting, for example, is conducted with a suppressed carrier AM-FM modulation system, generally called a pilot-tone system, which is modification of the suppressed carrier modulation system. FM stereo receivers for receiving such FM stereo broadcasting wave and reproducing the stereophonic sounds is designed so that the received broadcasting wave is FM-demodulated by an FM detector into the composite signal of a main-channel signal, a sub-channel signal and a pilot signal (19kHz), and this composite signal is then demodulated by a stereo adapter into signals of the left channel and the right channel. The stereo adapter described above is adapted to receive a carrier wave (38kHz), in addition to the main- and the sub-channel signals. This carrier wave is synchronous with the sub-carrier wave (38kHz) for the modulation of the sub-channel signal (the differential signal of the left- and right-channel signals) generated at the broadcasting station. In the conventional FM stereo receiver, the carrier wave which is used for demodulation of the received signal is usually obtained by doubling the frequency (19kHz) of the pilot signal extracted from the composite signal.
In order to accomplish this extraction of the pilot signal from the composite signal, it has been the practice to use a band-pass filter having such characteristics as shown in FIG. 1. As band-pass filter, there has been used in general an LC filter which is composed of a single or plural inductance element(s) and capacitance element(s). With an LC filter, however, the center frequency of the passing band thereof corresponding to the pilot signal frequency (fp: 19kHz) and the quality factor Q thereof tend to vary owing to the ambient temperature or for other reasons, so that the amplitude and the phase of the extracted pilot signal often tend to fluctuate. In addition, this band-pass filter is required to have a considerably high selectivity. However, the enhancement of its selectivity will, in turn, result in a steep phase-characteristic curve, and accordingly the phase of the extracted pilot signal will often tend to become unstable.